The present invention relates to a method for the conversion of variable physical quantities occurring in vehicles, represented as, frequencies and at least temporarily disturbed, into numerical values or signals proportional to the frequencies and suitable for immediate further processing. Moreover, the present invention is related to a circuit for use, in particular, with antiskid systems for automotive vehicles for implementing this method.
Typical examples of application possibilities of such a method are the fuel-injection control in a motor vehicle engine which is accomplished dependent on the rotational frequency of the crankshaft, the spark advance or retard control which is responsive to the same variable physical quantity, as well as antiskid control systems in which as a variable physical quantity the rotational speed of each individual wheel and time derivatives of this rotational speed, for example, the acceleration of each individual wheel, are sensed.
From German Patent DE-OS No. 2,519,867 a closed-loop control system is known which converts a signal, which is frequency-modulated in response to a variable physical quantity, into a value proportional to this physical quantity, the closed-loop control system comprising a comparator including an integrator driven by a subtractor and a dead-zone circuit, with the frequency-modulated signal being supplied to one input of the subtractor while the output signal of a filtering means inserted downstream from the comparator is supplied to the other input of the subtractor, and the dead-zone circuit issues three distinct output signals dependent on whether the integrator's output signal is below a predetermined lower threshold level, above a predetermined upper threshold level, or between these thresholds. In this arrangement, the filtering means comprises essentially a memory likewise operating as an integrator and a paralleled unit acting as a coefficient former having its output signal combined with the output signal of the memory by an adder stage generating the output signal of the filtering means. This known arrangement leaves much to be desired in terms of both its dynamic action and stability.
In order to obtain an optimum dynamic action in such arrangements, it is necessary to reach maxima in high resolution of time, quick response action and good stability of the system. Since these are contradicting requirements, compromises have to be sought which, however, must not lead to the introduction of instabilities into the system.